This study will determine the efficacy of the small molecule CDK4/6 inhibitor PD 0332991 (as measured by progression free survival at 6 months) in patients with recurrent glioblastoma multiforme or gliosarcoma who are Rb positive. A total of 30 patients will be treated; 15 will undergo a planned surgical resection and receive drug for 7 days prior to surgery, followed by drug after recovery from surgery, and the other 15 patients will receive drug without a planned surgical procedure.